The development of new and more energy efficient illumination devices is one of the important technical challenges which society faces. Common technologies which are more energy efficient than traditional lighting solutions are often based on Light Emitting Diodes (LEDs). High intensity light sources are interesting for many applications including spot lighting and digital light projection. For these purposes it is possible to use wavelength converters which convert light of a shorter wavelength, to light of a longer wavelength in a highly transparent luminescent material. In order to increase the intensity of the light emitted, light of a longer wavelength can then be extracted from just one surface of a wavelength converter.
However, in such applications it is important to effectively couple the light from a light source into the wavelength converter which often comprises transparent phosphor for providing wavelength conversion. Furthermore, it is desirable to maintain the generated light within the luminescent layer to avoid light loss from the points where LEDs are optically coupled to the luminescent layer. U.S. Pat. No. 7,982,229 describes a conversion structure comprising phosphor which receives light from blue LEDs, converts the light into longer-wavelength light and guides it to an exit surface, where the resulting brightness can be high. Furthermore it is suggested that there is no optical contact between the light source and the transparent luminescent material. In such a configuration, energy is dissipated within the luminescent element as light is converted to longer wavelengths. This leads to a considerable temperature increase within such an element if the heat produced is not transported away from the luminescent layer. Using an optical contact between the light source and the wavelength converter may improve the cooling such that the temperature is reduced, however an optical contact may also lead to losses of light generated within a wavelength converter through the optical contact to a surrounding medium.